Optical scanners are widely used for reading data encoded in symbols on various items. One common application of optical scanning is in reading of one-dimensional bar code symbols such as Universal Product Code (“UPC”) symbols and other symbols in which information is represented by a set of marks in the form of parallel lines and spaces between the marks. Optical scanners are also used to read two-dimensional bar code symbols such as the PDF417 code in which information is represented by a rectilinear pattern of marks and spaces in the form of blocks, such that the pattern as a whole resembles an irregular checkerboard. Most commonly, the marks are dark and hence have low reflectivity, whereas the spaces are light and hence have high reflectivity.
Optical scanners typically operate by directing a beam of light from a source such as a laser onto the object bearing the symbol and detecting the intensity of the reflected light. The scanner typically incorporates optical elements which focus the beam of light to a relatively small spot at the object bearing the symbol and which move the optical elements so as to sweep the spot of light in a predetermined scanning path across the object as, for example, in a series of parallel lines referred to as “raster”. These scanners also include a photodetector such as a photodiode or phototransistor which receives the light reflected from the object. As the spot of light moves over the object and encounters light and dark areas on the surface of the object, the photodetector is exposed to reflected light from the spot and hence is exposed to light reflected from points on object surface along the scanning path. The amount of light reflected to the photodetector varies with the reflectivity of the object surface at different points along the scanning path and the electrical signal produced by the photodetector varies correspondingly. Similar effects can be achieved by optical elements which limit the field of view of the photodetector to only a small spot so that the photodetector is exposed only to the light reflected by the object surface at the spot, and which sweep that spot along the desired scanning path. Some scanners use both techniques, so that both the illumination and the field of view of the photodetector are limited to the same spot, and that spot is swept along the scanning path.
The variations in the electrical signal from the photodetector typically are converted by known analog processing circuitry into a digital signal having a first or “space” value, (e.g., 0) when the spot is on a point having high reflectivity and having a second or “mark” value (e.g., 1) when the spot is focused on a point having low reflectivity. The digital values occurring at successive times represent the signal of the photodetector at successive times and hence representing the reflectivity of the object surface at successive points along the scanning path. The digital signal typically is converted by a unit referred to as a “digitizer” to a series of transition data elements, each such transition data element including data denoting the occurrence of a transition from the mark value to the space value or vice-versa, and the time interval since the previous transition. Each such time interval represents the width of a mark (dark region) or a space (light region) on the object being scanned. Because the time interval data commonly is obtained by counting cycles of a digital clock between transitions, the time interval data is commonly referred to as bar or space “count” data. These values are supplied to a decoder which uses known algorithms to recover from such values the information denoted by the symbol, such as numbers in the case of a UPC code.
Many bar code scanners perform several scans along different paths at different locations and/or at different orientations. For example, to read a one-dimensional bar code, the scanning path must extend across the bars rather than parallel to the bars. When objects bearing one-dimensional bar codes are presented in random orientation relative to the scanner, using several different scanning paths at different orientations relative to the scanner increases the probability that at least one scan path will be oriented correctly relative to the code on each object. Also, the scanner may provide scanning paths at different distances from the scanner, so that objects presented at different distances from the scanner can be successfully scanned.
Such multi-path scanners commonly use multiple photodetectors. For example, one holographic bar code laser scanner disclosed in commonly owned U.S. Pat. No. 5,984,185 uses multiple lasers and a separate photodiode associated with each laser. Similar scanners are disclosed in U.S. patent application Ser. No. 09/251,568, filed Feb. 17, 1999 and U.S. patent application Ser. No. 08/573,949 filed Dec. 18, 1995, now abandoned. The contents of the foregoing applications and patent are incorporated herein by reference.
Typically, a separate set of components including a photodiode, an analog processing circuit, digitizer and decoder is associated with each laser. Such a set of components is referred to as a “channel”. The decoded information from each channel is then provided as input to a multiplexing microprocessor to derive a single output. The owner of the present application has practiced this method of signal detection, processing and decoding in its HoloTrak (registered trademark) line of holographic bar code scanners.
This approach requires that each channel have a complete set of separate circuitry, including a separate decoder for each channel. Moreover, an additional microprocessor or other multiplexing device must be provided to combine the decoded information from the plural channels into a single stream of decoded data for delivery to a host computer or other device which uses the decoded information. These factors add to the size and cost of the scanner. Thus, still further improvement would be desirable.